Whenever two or more drugs are being taken, there is a chance that there will be an interaction among the drugs. The interaction may increase or decrease the effectiveness of the drugs or their side effects. The likelihood of drug interactions increases as the number of drugs being taken increases. Therefore, people who take many drugs are at the greatest risk for interactions. Drug interactions contribute to the cost of healthcare because of the costs of medical care that are required to treat problems caused by changes in effectiveness or side effects. Interactions also can lead to psychological suffering. This review discusses the issue of drug interactions and several ways to avoid them.

A drug interaction can be defined as an interaction between a drug and another substance that prevents the drug from performing as expected. This definition applies to interactions of drugs with other drugs (drug-drug interactions), as well as drugs with food (drug-food interactions) and other substances, such as supplements. Drugs also may interact with laboratory tests, changing the proper results of the laboratory test.

There are several mechanisms by which drugs interact with other drugs, food, and other substances. An interaction can result when there is an increase or decrease in:

1. the absorption of a drug into the body;
2. distribution of the drug within the body;
3. alterations made to the drug by the body (metabolism); and
4. elimination of the drug from the body.

Most of the important drug interactions result from a change in the absorption, metabolism, or elimination of a drug. Drug interactions also may occur when two drugs that have similar (additive) effects or opposite (canceling) effects on the body are administered together. For example, there may be major sedation when two drugs that can cause sedation are taken simultaneously, such as narcotics with antihistamines.

Another source of drug interactions occurs when one drug alters the concentration of a substance that is normally present in the body. The alteration of this substance reduces or enhances the effect of another drug that is being taken. The drug interaction between warfarin (Coumadin) and vitamin K-containing products is a good example of this type of interaction. Warfarin acts by reducing the concentration of the active form of vitamin K in the body. Therefore, when vitamin K is taken, it reduces the effectiveness of warfarin.

Most drugs are absorbed into the blood and then travel to their site of action. Most drug interactions that are due to altered absorption occur in the intestine. There are various ways that the absorption of drugs can be reduced. These mechanisms include:

1. an alteration in blood flow to the intestine;
2. change in drug metabolism (breakdown) by the intestine;
3. increased or decreased intestinal motility (movement);
4. alterations in stomach acidity, and
5. a change in the bacteria that normally reside in the intestine.

Drug absorption also can be affected if the drug's ability to dissolve (solubility) is changed by another drug or if a substance (for example, food) binds to the drug and prevents its absorption.

Most drugs are eliminated through the kidney in either an unchanged form or metabolized by the liver. Therefore, the kidney and the liver are very important sites of potential drug interactions. Some drugs are able to reduce or increase the metabolism of other drugs by the liver or their elimination by the kidney.

Metabolism of drugs is the process through which the body converts (alters or modifies) drugs into forms that are more or less active (for example, by converting drugs that are given in inactive forms into their active forms that actually produce the desired effect) or that are easier for the body to eliminate through the kidneys. Most drug metabolism takes place in the liver, but other organs also may play a role (for example, the kidneys, intestine, etc.). The cytochrome P450 enzymes are a group of enzymes in the liver that are responsible for the metabolism of most drugs. They are, therefore, often involved in drug interactions. Drugs and certain types of food may increase or decrease the activity of these enzymes and therefore affect the concentration of drugs that are metabolized by these enzymes. An increase in the activity of these enzymes leads to a decrease in the concentration and effect of an administered drug. Conversely, a decrease in enzyme activity leads to an increase in drug concentration and effect.

Drug interactions may lead to an increase or decrease in the beneficial or the adverse effects of the given drugs. When a drug interaction increases the benefit of the administered drugs without increasing side effects, both drugs may be combined to increase the control of the condition that is being treated. For example, drugs that reduce blood pressure by different mechanisms may be combined because the blood pressure lowering effect achieved by both drugs may be better than with either drug alone.

The absorption of some drugs is increased by food. Therefore, these drugs are taken with food in order to increase their concentration in the body and, ultimately, their effect. Conversely, when a drug's absorption is reduced by food, the drug is taken on an empty stomach.

Drug interactions that are of greatest concern are those that reduce the desired effects or increase the adverse effects of the drugs. Drugs that reduce the absorption or increase the metabolism or elimination of other drugs tend to reduce the effects of the other drugs. This may lead to failure of therapy or warrant an increase in the dose of the affected drug. Conversely, drugs that increase absorption or reduce the elimination or metabolism of other drugs - increase the concentration of the other drugs in the body - and lead to increased amounts of drug in the body and more side effects. Sometimes, drugs interact because they produce similar side effects. Thus, when two drugs that produce similar side effects are combined, the frequency and severity of the side effect are increased.

The prescribing information for most drugs contains a list of potential drug interactions. Many of the listed interactions may be rare, minor, or only occur under specific conditions and may not be important. Drug interactions that cause important changes in the action of a drug are of greatest concern.

Drug interactions are complex and often unpredictable. A known interaction may not occur in every individual. This can be explained because there are several factors that affect the likelihood that a known interaction will occur. These factors include differences among individuals in their:

• genes,
• physiology,
• age,
• lifestyle (diet, exercise),
• hydration,
• underlying diseases,
• drug doses,
• the duration of combined therapy, and
• the relative time of administration of the two substances. (Sometimes, interactions can be avoided if two drugs are taken at different times.)

Nevertheless, important drug interactions occur frequently and they add millions of dollars to the cost of health care. Moreover, many drugs have been withdrawn from the market because of their potential to interact with other drugs and cause serious health care problems.

1. Give healthcare professionals a complete list of all of the drugs that you are using or have used within the last few weeks. This should include over-the-counter medications, vitamins, food supplements, and herbal remedies.
2. Inform healthcare professionals when medications are added or discontinued.
3. Inform healthcare professionals about changes in lifestyle (for example, exercise, diet, alcohol intake.
4. Ask your healthcare professionals about the most serious or frequent drug interactions with the medications that you are taking.
5. Since the frequency of drug interactions increases with the number of medications, work with your healthcare professionals to eliminate unnecessary medications. Always discuss potential drug interactions with our pharmacist.

This brief overview of drug interactions does not cover every possible scenario. Individuals should not be afraid to use their drugs because of the potential for drug interactions. Rather, they should use the information that is available to them to minimize the risk of such interactions and to improve the success of their therapy.